1. Field
The presently disclosed subject matter relates to LED lighting units, and more particularly to LED lighting units having a high color rendering of which a color temperature can arbitrarily adjust such that it is close to a natural color wherein the lighting unit also has a simple structure.
2. Description of the Related Art
Various LED lighting units that include LEDs as a light source have been developed in recent years. One reason is that LEDs may be a favorable light source with respect to saving energy, environmental problems, etc. In addition, LEDs can emit variously-colored lights while the structure of the LED can be thin and small in size.
A method for emitting white light by using LEDs as a light source will now be described. An LED lighting unit for emitting white light is composed of a red LED, a green LED and a blue LED as a light source. The LED lighting unit can emit white light by additive color mixture in which red, green and blue light of the three additive primary colors are mixed with respect to each other.
A conventional LED lighting unit that emits white light by additive color mixture, for example, is disclosed in Patent Document No. 1 (Japanese Patent Application Laid Open JP2001-184910). An LED lighting unit disclosed in Patent Document No. 1 can emit white light by controlling each beam ratio of red, green and blue lights and mixing their lights with respect to one another.
Specifically, a red LED having both a peak wavelength λp of 605 nm to 635 nm and a half bandwidth Δλ of 15 nm to 60 nm in emission spectrum, a green LED having both a peak wavelength λp of 530 nm to 570 nm and a half bandwidth Δλ of 20 nm to 60 nm in emission spectrum and a blue LED having both a peak wavelength λ p of 450 nm to 490 nm and a half bandwidth Δλ of 15 nm to 70 nm in emission spectrum are used as light sources for the LED lighting unit.
In this case, the LED lighting unit can emit various types of white lights having both a correlated color temperature and a general color rendering index by changing each beam ratio of red, green and blue lights. FIG. 11 is a spectrum distribution diagram showing a spectrum of mixed white light made according to the conventional method of additive color mixture.
A white light A shown in FIG. 11 has both correlated color temperature of 6,500 K and general color rendering index Ra of 95.2 by the additive color mixture in which each beam ratio of a red light having both a peak wavelength λp of 620 nm and a half bandwidth Δλ of 16.1 nm in emission spectrum, a green light having both a peak wavelength λp of 550 nm and a half bandwidth Δλ of 48.0 nm in emission spectrum, and a blue light having both a peak wavelength λp of 470 nm and a half bandwidth Δλ of 68.8 nm in emission spectrum, is 20.67, 54.17 and 25.16 percent, respectively.
A white light B shown in FIG. 11 has both correlated color temperature of 5,000 K and general color rendering index Ra of 94.4 by the additive color mixture in which each beam ratio of the red, green and blue lights is 22.88, 57.89 and 19.24 percent, respectively. A white light C shown in FIG. 11 has both correlated color temperature of 3,000 K and general color rendering index Ra of 91.3 by the additive color mixture in which each beam ratio of the red, green and blue lights is 32.12, 59.35 and 8.24 percent, respectively.
Another method for emitting white light by using LEDs as a light source will now be given. Another LED lighting unit for emitting white light is composed of a blue LED as a light source and at least one phosphor for wavelength-converting a part of blue light emitted from the blue LED.
For instance, by using the blue light emitted from the blue LED and red and green lights that are made by converting a wavelength of blue light emitted from the blue LED, the LED lighting unit can emit white light by an additive color mixture in which red, green and blue light of the three additive primary colors are mixed with respect to each other.
When the red light is made by the blue LED and a phosphor, the blue light emitted from the blue LED is converted to a wavelength of red light by exciting a red phosphor with the blue light. Similarly, when the green light is made by the blue LED and a phosphor, a green phosphor is excited by the blue light emitted from the blue LED and the blue light is converted to a wavelength of green light. Thus, the LED lighting unit includes a blue LED as a light source and red and green phosphors for converting the blue light emitted from the blue LED into red and green wavelength light.
Another conventional LED lighting unit that emits white light by the above-described additive color mixture, for example, is disclosed in Patent Document No. 2 (Japanese Patent Application Laid Open JP2002-60747). An LED lighting unit disclosed in Patent Document No. 2 can emit white light by controlling each beam ratio of a blue light emitted from a blue LED and red and green light that is wavelength-converted from the blue light via phosphors and by mixing their light with respect to each another.
Specifically, the LED lighting unit includes a blue LED having a peak wavelength λp of 460 nm and a mixture phosphor including both a red phosphor that is composed of SrS:Eu and a green phosphor that is composed of SrGa2S4:Eu. The mixture phosphor allows a part of the blue light emitted from the blue LED to convert to red and green light, and therefore the LED lighting unit can emit white light by the additive color mixture in which a non-converted blue light and wavelength-converted red and green light are mixed with respect to each other.
In this case, the LED lighting unit can emit various types of white light having both a correlated color temperature and a general color rendering index by changing each beam ratio of red, green and blue light, such as by controlling a quantity of blue light emitted from the blue LED and each quantity of red and green phosphors, etc. FIG. 12 is a spectrum distribution diagram showing a spectrum of mixed white light made according to the above-described other conventional method of additive color mixture.
For instance, a white light D shown in FIG. 12 has both correlated color temperature of 3,000 K and general color rendering index Ra of 94. Similarly, white lights E, F and G shown in FIG. 12 have both correlated color temperature of 3,800 K, 4,400K and 4,900K and general color rendering index Ra of 94, 94 and 92, respectively.
The above-referenced Patent Documents are listed below.    1. Patent Document No. 1: Japanese Patent Application Laid Open JP2001-184910    2. Patent Document No. 2: Japanese Patent Application Laid Open JP2002-60747
As described above, the method for emitting white light by using red, green and blue LEDs is disclosed in Patent Document No. 1. However, the LED lighting unit disclosed in Patent Document No. 1 may emit all color lights including white light by controlling each beam ratio of red, green and blue light and mixing the different wavelength light with respect to one another.
More specifically, the above-described all color lights means all colors that are located within a triangle of an xy chromaticity diagram in which three apexes are each of light emitted from the red, green and blue LEDs. Vector synthesis of the three single wavelengths can result in emitting all the color lights because the red, green and blue LEDs have spectrum distributions close to the respective single wavelengths.
However, each emission spectrum of red, green and blue light emitted from the LEDs of the above-described LED lighting unit has a narrow half bandwidth, respectively. In addition, each emission spectrum of red, green and blue light cannot maintain a contiguous wavelength area between each emission spectrum thereof due to each separate wavelength component thereof.
Thus, for example, when controlling the white light by an additive color mixture in which red, green and blue light of the three additive primary colors are mixed with respect to each other, and when emitting the mixture light to the outside via a lens, the mixture light may cause each separate area of wavelength components at a contoured part of radiation range because a refractive index of lens varies from each wavelength component of red, green and blue light. Consequently, it may be difficult for the conventional method to maintain a uniform color within the whole radiation range.
To help solve the problem, the LED lighting unit uses an optical mixing structure that may be a relatively complex optical system. In this case, the optical mixing structure may need a large size of LED lighting unit because of each alignment of the LEDs, each long light-path, etc. In addition, when at least one peak wavelength of the emission spectrums of red, green and blue light shifts, the change of noncontiguous wavelength may cause a large fluctuation of general color rendering index Ra in the mixed color light.
Moreover, each beam of red, green and blue light may be small because each integration value of spectrum components of a respective light is small. Especially, each beam of red and blue light having low luminosity factors may extremely decrease with respect to a beam of green light having a high luminosity factor. Thus, it may be difficult for the LED lighting unit to emit brightly due to a low beam of mixed color light.
Likewise, the other method for emitting white light by controlling each beam ratio of a blue light emitted from a blue LED and red and green light that is wavelength-converted from the blue light can have similar problems as referenced with regard to the above-described method.
That is to say, because the blue light emitted from the blue LED and the red and green light that is wavelength-converted from the blue light are mixed with respect to each other, the mixture light emitted to the outside via a lens may cause separation of each area of the wavelength components at a contoured part of a radiation range because a refractive index of lens varies from each wavelength component of red, green and blue light. Thus, the other method may also be difficult to maintain a uniform color within the whole radiation range.
In addition, the other method may be difficult to control peak wavelengths of emission spectrum of red and green light within a normal range of variation via the mixture phosphor. When a peak wavelength of blue light emitted from the blue LED shifts, all the peak wavelengths of light shift and therefore the changes of wavelengths may cause a fluctuation of general color rendering index Ra in the mixed color light.
The disclosed subject matter has been devised to consider the above and other problems and characteristics. Thus, embodiments of the disclosed subject matter can include LED lighting units having a high brightness and a high color rendering that can maintain a uniform color within the whole radiation range.
Furthermore, the disclosed subject matter can also include an LED lighting unit of which a color temperature of emission color is configured to arbitrarily adjust close to a natural color such as sunlight, and can reduce or change associated problems and characteristics of the conventional lighting units and methods.